Diglyceride is a product obtained by the esterification of one glycerin (glycerol) molecule with two fatty acid molecules. It is a natural ingredient in oils and fats, but its content in the edible oil is often low, generally less than 10%. It is an important member of polyol-type nonionic surfactants and has various advantages, such as good safety, high nutrition value, better processing adaptability, high compatibility with the human body, etc. Therefore, it is a kind of multifunctional additive with a wide range of applications in food, medicine and chemical (cosmetics) industries. At the same time, diglyceride has a unique physiological activity—it can inhibit the diseases, such as obesity, hypertension, heart disease, etc. Therefore, it has attracted a lot of attentions in recent years.
Diglyceride can be obtained by chemical methods, but in recent years, enzymatic methods are also used because of their pros including mild reaction conditions, high specificity, and being environment-friendly. A large number of studies have been conducted on the methods of synthesizing diglyceride by enzymatic reaction in batch modes, e.g., currently the most well-studied one is the batch kettle reactor. In addition, the method for continuous production of diglyceride, i.e. production of diglyceride by a packed column reactor, also attracts a lot of attentions.
As for the batch reactions, the substrate and the biocatalyst (generally immobilized enzyme) are added into the reactor. After reaction, the product is taken out, and the catalyst enzyme is collected by filtration for reuse. However, the enzyme particle used in this method cannot be packed densely. Also, the reaction process is accompanied with strong stirring, and the shear force generated by stirring can destroy the enzyme protein, denaturing and inactivating the enzyme and thus reduces the service life.
As for the packed-bed reactor, the biocatalyst immobilized enzyme is packed uniformly in the reactor, then the reaction substrate is introduced into the reactor in a plug flow form at a volume flow rate, after circulation and reaction are carried out for a certain period of time, the reaction product is collected. As there are no back-mixing in the reactor, it is suitable for the continuous production of biological enzymatic conversion of the immobilized enzyme and the low viscosity reaction substrate. However, in this system, as the reaction proceeds, water, as a by-product, will be accumulated gradually in the reactor, which not only can reduce the esterification rate, but also hydrolyze the esterification product because water will also compete with glycerol for the acylase intermediate. In addition, the excess water will be adsorbed into the immobilized enzyme, thus reducing the activity of the enzyme. The reactor is suitable for the reaction in a homogeneous system, but is not suitable for a reaction in a heterogeneous system, because the two-phase stirring can not be carried out in the packed bed.